The present invention relates to temperature compensated surface acoustic wave (SAW) devices.
SAW devices utilize the localized propagation of acoustic waves on the surface of a planar piezoelectric substrate. SAW transduction between electrical signals and acoustic waves is accomplished by thin film metallic interdigital electrodes on the substrate surface. SAW propagation velocity is temperature sensitive, but SAW devices must often work over a wide temperature range, so devices may be mounted in a custom oven to maintain a fixed temperature above the maximum ambient temperature.
An oven comprises a device holder, heater, temperature sensor, feedback temperature controller such as operational amplifier, thermal insulation, and electrical connections between the device and ambient. An oven contains (and is thus larger than) the ovenized device and consumes significant power.
One example of an attempt to provide more efficient temperature compensation for a SAW device, is described in U.S. Publication 2008/0055022A1. The SAW substrate is contained within a vacuum housing which in turn is within a packaging, and a heater is located on the housing or the bottom of the SAW substrate, opposite the acoustic propagation surface. Although a distinct oven around the packaging is avoided, the heater is still remote from the propagation surface of the SAW substrate.
Pending U.S. application Ser. No. 13/065,177 for “Monolithically Applied Heating Elements on SAW Substrate” discloses a “micro-oven” technique for heating and preferably temperature sensing only the localized surface where the surface acoustic waves actually exist. The heater and preferably associated temperature sensor are realized as thin film metallic meander resistor electrodes on the substrate propagation surface, which can be deposited monolithically with the transducers and other functional features from the same photomask and photolithographic manufacturing process. One embodiment is directed to a surface SAW device comprising a substrate having a working surface with an active zone capable of propagating an acoustic wave, at least one interdigital transducer on the working surface, and a heating element on the working surface, adjacent to at least the active zone, wherein the transducer and heating element have the same material composition.
Although this monolithic heating system represents a major improvement over previous systems, a significant amount of heat generated by the system is not applied to the SAW surface due to connection losses.